About 40 IAM members filed complaints with the NLRB, alleging that the headquarters in Upper Marlboro, Maryland, scheduled the vote to undercut opposition to the contract. Many senior members, who were thought more likely to vote no, were on holiday vacation that day.

The offer cut negotiated benefits, including moving workers’ retirement benefits from a traditional defined pension plan to a defined contribution plan. In return for approving the contract, Boeing promised to place final assembly and wing fabrication for the new 777X airliner in metro Puget Sound.

Members of IAM District Lodge 751, which represents about 31,000 Boeing employees, narrowly approved the contract.

But a vote was never actually even necessary. The IAM constitution actually gives the headquarters authority to accept a contract without a vote, according to a letter sent by the NLRB to Robley Evans, who filed one of the complaints. He is president of Local F of District Lodge 751 and a forklift operator at Boeing’s Auburn plant.

Evans said he plans to appeal.

“The IAM can accept a contract and ram it down our throats, but once they decide to hold a vote, I believe they have a responsibility to give members a fair vote.”

The letters were sent by the NLRB’s Seattle office, where a spokesman could not immediately be reached for comment.

Boeing has “not received any formal notification from the NLRB regarding the status of these charges,” company spokesman Doug Alder said. “We continue to move forward implementing all terms of the contract, including the major expansion of our Everett site for the 777X.”